The mammalian gut houses a complex and diverse microbial ecosystem in which microbial members establish commensal, symbiotic, and sometimes pathogenic relationships with their mammalian host. In order to colonize this densely populated environment, these bacteria must also establish both mutualistic and antagonistic relationships with other microbial inhabitants. Recent studies have examined the beneficial properties conferred to the host by members of the intestinal microbiota, but few studies have examined the important relationships and interactions that exist between the microbial members of this ecosystem. Bacteroides spp. are abundant members of the human intestinal microbiota, accounting for approximately 20% of the total organisms of this ecosystem. Our long term goals are directed toward elucidating microbial factors and mechanisms that allow Bacteroides spp. to colonize and persist in the mammalian intestine where they provide benefits to the host. We are also interested in understanding the microbial factors that contribute to microbial diversity in ths ecosystem and how we may exploit antimicrobial molecules produced by these bacteria for therapeutic purposes or to manipulate the composition of the ecosystem to positively impact human health. This application addresses the production of bacterially-encoded antimicrobial molecules, known as bacteriocins. Bacteriocin production by intestinal Bacteroides spp. was reported many years ago, however, no bacteriocin gene was cloned, nor was the mechanisms of action described. In addition, the contribution of bacteriocin production to population dynamics and microbial diversity in the intestinal ecosystem has not been explored. The experiments outlined in this proposal will identify and characterize the numerous bacteriocins produced by a Bacteroides fragilis type strain at the molecular level to understand the breadth and types of bacteriocins produced by this order of bacteria. Aim 2 includes more in-depth analyses of a unique bacteriocin with a membrane attack complex motif contained on molecules produced by mammalian immune cells. Similar molecules are encoded by the genomes of diverse Bacteroidetes species and this bacteriocin may be a common defensive/offensive molecule used by these Bacteroidetes species, most of which inhabit diverse polymicrobial ecosystems. For the third aim, we will perform well-controlled ecological studies using the MACPF bacteriocin as a model. In vitro systems representing both spatially structured and unstructured communities will be analyzed. In addition, we will perform experiments using a gnotobiotic mouse intestinal colonization model to determine the relevance of this bacteriocin in the natural mammalian intestinal ecosystem. These experiments will determine if this bacteriocin promotes microbial diversity, if it facilitates the invasion of a strain into an established ecosystem, and its role in thwarting colonization of a competing sensitive strain. This combination of studies will provide a comprehensive analysis of these antimicrobial molecules and the significance of their production in the dense and competitive intestinal ecosystem.